1. Field of the Invention
The present invention relates to a transparent substrate, used in a phase-shift mask for manufacturing semiconductor, a mask for processing optical products, or other optical products, having a character/symbol section in which a slit-shaped or lattice-shaped pattern comprising concave grooves is provided on one surface of the substrate only in a prescribed area in response to the shapes of the characters and/or the symbols, and a manufacturing method thereof.
2. Description of the Related Art
There is recently an increasing demand for a higher density and a finer configuration of semiconductor chip circuits. It is therefore necessary to transfer a finer circuit pattern onto a resist on a wafer. From such a point of view, the conventional binary mask forming a circuit pattern with a light shielding section and a transparent section is insufficient to satisfy requirements.
More recently, on the other hand, progress in optical lithographic technology is really remarkable, and as a technique for improving resolution of the projecting exposure unit, it is the present common practice to adopt the phase-shift process which increases the pattern resolution by using different phases of light passing through two adjacent transparent portions on the photomask.
Various types of phase-shift mask used in this phase-shift process have been developed, and are used in response to the purposes of use. Among others, the engraving type phase-shift mask of forming a shifter section by engraving the substrate is becoming more common.
A substrate-engraving-type chromeless phase-shift mask for processing optical products is now applied for processing optical fibers.
Because a layer of a chromium-based metal such as chromium, chromium oxide or chromium oxide nitride is usually used as a light shielding layer for various phase-shift masks, a phase-shift mask which does not have a light shielding layer but uses a shifter section comprising concave grooves formed on a surface of a transparent substrate is referred to as a chromeless phase-shift mask of the substrate-engraving type.
In the above-mentioned photomask, it is required to provide characters or symbols. It therefore becomes necessary to conduct operations for forming characters and symbols on the mask, together with preparation of a circuit pattern on the mask portion, thus requiring more and more complicated manufacture of the mask as a whole.
It is demanded, not to perform formation of characters and symbols as an independent and separate step, but to form characters and symbols within a photomask preparing step as far as possible. In the conventional manufacturing process of a substrate-engraving-type chromeless phase-shift mask, the problems shown in FIG. 3 are encountered.
FIG. 3(a) is a schematic plan view of the middle of processing of a conventional substrate-engraving-type chromeless phase-shift mask having characters and symbols; FIG. 3(b) is a schematic view illustrating enlarged characters in B1 portion of FIG. 3(a); FIG. 3(c) is an enlarged view of B2 portion shown in FIG. 3(b); FIG. 3(d) is a sectional view of FIG. 3(c) cut along the line B3-B4; and FIGS. 3(c1) and 3(d1) illustrate states after processing corresponding to FIGS. 3(c) and 3(d), respectively. In FIG. 3, reference numeral 200 represents a phase-shift mask; 210 represents a transparent substrate; 220 represents a main pattern section; 230 represents a character/symbol section; 231 represents a light shielding film; 232 represents an opening (of the light shielding film); and 215 represents a concave groove section.
Character “2” in the B1 portion of the character/symbol section 230 shown in FIG. 3(a) is formed by opening the light shielding film 231, as shown in FIG. 3(b). An enlarged schematic plan view of B2 portion thereof is as shown in FIG. 3(c). The sectional view of FIG. 3(c) cut along the line B3–B4 is as shown in FIG. 3(d).
The conventional manufacture of a substrate of a substrate-engraving-type chromeless phase-shift mask is accomplished simultaneously with the forming step of a circuit pattern which is the main pattern for the formation of a character/symbol section, resulting in the manner shown above.
However, this manufacturing method of the substrate-engraving-type chromeless phase-shift mask contains a step of wet-etching all or part of the light shielding film, following the forming step of characters and symbols, and patterned characters in this step sometimes disappear as shown in FIGS. 3(c1) and 3(d1).
A counter-measure was taken to prevent disappearance of patterned characters and symbols by a method comprising the steps of covering the characters and symbols with a wet-etching-resistant protective film and carrying out wet etching. In this case, however, problems were encountered in that excessive light shielding film remained near the characters, and the addition of a coating step of the protective film led to a lower operating efficiency.
The present invention was developed to solve the problems as described above, and has an object to provide a substrate-engraving-type chromeless phase-shift mask which poses no problem in quality, gives a high operating efficiency, and permits arrangement of characters and symbols, and a manufacturing method thereof.